Ion implantation is a standard technique for introducing property-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the sub-surface of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity or material property.
High dose implantation may allow the lowest cost-of-ownership for an ion implanter. Localized or selective doping or localized or selective material modification may be required for some implants. Fabrication of solar cells presents one example in which high dose implantation and selective doping of local areas is desirable. Doping, which may improve efficiency of solar cells, may be performed using ion implantation. FIG. 1 is a cross-sectional view of a selective emitter solar cell 10. It may increase efficiency (the percentage of light converted to electrical energy) of a solar cell to dope the emitter 200 and provide additional dopant to the regions 201 under the contacts 202. More heavily doping the regions 201 improves conductivity and having less doping between the contacts 202 improves charge collection. The contacts 202 may only be spaced approximately 2-3 mm apart. The regions 201 may only be approximately 100-300 μm across. The solar cell 10 may also include an ARC layer 22, disposed above the emitter 200 and a base layer 24, as well as backside contact 26. FIG. 2 is a cross-sectional view of an interdigitated back contact (IBC) solar cell 20. In the IBC solar cell 20, the junction is on the back of the solar cell. The solar cell may have an arc layer 20, passivating layer 28, and N+ front surface field 30 that form a stack adjacent an N-type base layer 32. The doping pattern may include alternating p-type and n-type dopant regions in this particular example. The p+ emitter 203 and the n+ back surface field 204 may be doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency. The p-type contact fingers 34 and n-type contact fingers 36 may be formed in contact through holes 38 formed in passivating layer 40.
In manufacturing articles such as solar cells, the use of known patterning processes, such as photolithography, in conjunction with implantation, may be too cost prohibitive for use to perform selective area implantation because of the extra steps required.
Plasma doping technology is not fully tested for such applications. Direct exposure to neutrals in the plasma may cause deposition or etching of a workpiece and may require additional cleaning steps. Accordingly, there is a need in the art for an improved implantation of workpieces and, more particularly, to an improved method and apparatus for patterned implantation of workpieces without the use of masks.